1. Field of the Invention
The present invention relates to a membrane/electrode assembly for polymer electrolyte fuel cells, and a polymer electrolyte fuel cell.
2. Discussion of Background
Fuel cells are expected to be widely used, since is their power generation efficiency is high, the reaction product is only water in principle, and the load to the environment is small. Among them, a polymer electrolyte fuel cell has a high output density and is therefore expected to be widely used as an automobile or distributed power generation system, a portable power generation system for a household cogeneration system.
A polymer electrolyte fuel cell is usually constituted by a cell wherein an electrically conductive separator having gas flow paths formed, is disposed on each side of a membrane/electrode assembly comprising a cathode having a catalyst layer and a gas diffusion layer, an anode having a catalyst layer and a gas diffusion layer, and a polymer electrolyte membrane disposed between the catalyst layer of the cathode and the catalyst layer of the anode.
In a case where the dimensional stability or the mechanical strength of the membrane/electrode assembly is low, the handling efficiency is likely to be poor in assembling the cell, or the polymer electrolyte membrane is likely break during its operation. Therefore, the membrane/electrode assembly is required to have sufficient mechanical strength and dimensional stability.
Further, recently, the polymer electrolyte fuel cell is required to be operated under a low humidity condition where the relative humidity of the reaction gas (fuel gas and oxidant gas) is low, in order to simplify the fuel cell system or to reduce the cost. If power generation can be carried out constantly under a low humidity condition, it will be unnecessary to provide a peripheral device such as a humidifying device, whereby it is possible to reduce the size or the costs of the fuel cell system. Accordingly, the polymer electrolyte membrane for the membrane/electrode assembly is required to have a high ion exchange capacity (i.e. the equivalent weight (grams of the polymer per equivalent of ionic groups, hereinafter referred to as EW) being small) and a thin thickness (at most 25 μm) in order to maintain the ion conductivity even under a low humidity condition.
However, the polymer electrolyte membrane has such a nature that as EW is smaller, it tends to undergo swelling and shrinkage due to changes of the humidified environment. Such swelling and shrinkage occur due to changes of the operation conditions such as the cell temperature, the relative humidity of the reaction gas, the amount of the reaction gas, the output, etc., and in a practical application, the polymer electrolyte membrane undergoes dimensional changes disorderly as the swelling and shrinkage are repeated. Consequently, the polymer electrolyte membrane will get wrinkles. And, in a case where the thickness of the polymer electrolyte membrane is thin, the polymer electrolyte membrane may break due to such wrinkles.
The following ones have, for example, been proposed as a polymer electrolyte membrane and a membrane/electrode assembly having the dimensional stability improved.
(1) A thin composite membrane having a thickness of at most about 25 μm having an ion exchange resin impregnated to a stretch-expanded tetrafluoroethylene film having a porous fine structure (Patent Document 1).
(2) A composite membrane having an ion conductive polymer contained in porous bodies of individual fibers randomly oriented (Patent Document 2).
(3) A membrane/electrode assembly having a reinforcing material containing electrically conductive nano fibers disposed on at least one side of a polymer electrolyte membrane (Patent Document 3).
However, the composite membrane (1) has such a problem that as compared with a membrane not reinforced, the ion conductivity tends to be low, and the power generation performance tends to be low particularly under a low humidity condition.
The composite membrane (2) also has a problem such that when a porous material having sufficient chemical stability and mass productivity is selected for use, the ion conductivity tends to be low as compared with the membrane not reinforced, and the power generation performance tends to be low particularly under a low humidity condition.
In the case of the membrane/electrode assembly (3), the dimensional stability and mechanical strength are still inadequate, and it is not durable against the above-mentioned repetition of swelling and shrinkage especially when the thickness of the polymer electrolyte membrane is at most 25 μm.
Patent Document 1: U.S. Pat. No. 5,547,551
Patent Document 2: JP-A-10-312815
Patent Document 3: JP-A-2006-252967